Bundlephobia
bundlephobia.com
Shows what an npm package costs your bundle before you install it
No sign-up
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Format, validate, convert, test and share code.
bundlephobia.com
Shows what an npm package costs your bundle before you install it
caniuse.com
Answers whether a browser feature is safe to ship yet
codepen.io
Write HTML, CSS and JavaScript in a tab and watch it render live
crontab.guru
Translates a cron expression into a plain English schedule as you type
gchq.github.io
Chains encoding, hashing and decryption steps into a repeatable recipe
devdocs.io
One fast search box across a hundred API references, usable offline
diffchecker.com
Shows exactly what changed between two blocks of text, side by side
hoppscotch.io
Sends HTTP, GraphQL and WebSocket requests from a browser tab
jsoncrack.com
Turns a wall of JSON into a node graph you can pan and search
jwt.io
Decodes a JSON Web Token into readable header, claims and signature
onecompiler.com
Compiles and runs code in around seventy languages with no sign-up
pastebin.com
Drops a block of text or code at a short URL you can send to anyone
regex101.com
Builds and explains a regular expression while you type it
sqlformat.org
Reformats an unreadable SQL statement into something reviewable
stackblitz.com
Runs npm install and a Node dev server inside the browser itself
uuidgenerator.net
Hands you a fresh UUID on page load, single or in bulk
Privacy
These process everything inside the browser. Load the page, disconnect from the network, and they still work.
gchq.github.io
Chains encoding, hashing and decryption steps into a repeatable recipe
devdocs.io
One fast search box across a hundred API references, usable offline
hoppscotch.io
Sends HTTP, GraphQL and WebSocket requests from a browser tab
jsoncrack.com
Turns a wall of JSON into a node graph you can pan and search
jwt.io
Decodes a JSON Web Token into readable header, claims and signature
sqlformat.org
Reformats an unreadable SQL statement into something reviewable
stackblitz.com
Runs npm install and a Node dev server inside the browser itself
By task
Archive
Kept on record so an old bookmark still leads to an answer. Each one names the site that does the job now.
yahoo.com
Wired feeds and APIs together on a visual canvas, with no code
Questions
There is no single answer, which is why this page lists 16 of them with their limits stated. If you want to avoid signing up entirely, 11 of these work with no account at all.
7 of the 16 sites here process your file inside the browser and never upload it — they are marked "On device". The rest send your input to their server, which is fine for public material and worth thinking twice about for anything personal.
Some already have. 1 entry in this category has been retired and moved to the Archive with a working successor, so an old bookmark still leads somewhere useful.